Improving Rotary Screw Compressor Performance using Variable

Improving Rotary Screw Compressor Performance using Variable

Improving Rotary Screw Compressor
Performance using Variable Speed Drives
John Cosner, JCI/Frick
A Global Energy Situation….
Efficiency In The Engine Room
Optimum
Constant Speed
@ 100% Cap.
Constant Speed
@ 100% Cap.
VFD @ 30% Cap.
S.V. Loaded
Constant Speed
@ 100% Cap.
Constant Speed
@ 100% Cap.
Constant Speed
@ 30% Cap.
Constant Speed
@ 100% Cap.
Constant Speed
@ 70% Cap.
Better
Inefficient
Constant Speed
@ 60% Cap.
Compressor Facts
• Often the most concentrated and
largest consumers
• Most new installations use rotary
screw compressors.
• At least one compressor will go
to a part load condition.
Constant Torque / Variable Horsepower
Torque and Power
Screw Compressors
100%
80% 460V / 60Hz = 7.67V : 1Hz
60%
40%
20%
0%
0%
Horsepower
Torque
20%
Constant Torque
Horsepower = % Speed
40%
60%
Speed
80%
100%
Full Load Power = 500HP / 375KW
FLA = 580
Running @ 35% Capacity (by speed)
VSD
Voltage = 161Vpwm
HP = 175
Amps = 580
35% Out
21 Hertz
1260 RPM
Voltage = 460vac
HP = 175
Amps = 203
580 Amps
132 KW
Motor
Variable Torque / Variable Horsepower
Pumps & Fans
Torque and Power
100%
80%
60%
40%
20%
Horsepower
Torque
0%
0%
20%
40%
60%
Speed
Torque = % Speed²
Horsepower = % Speed³
80%
100%
Part Load Efficiencies @ Approximately 6:1 CR
% Full Load Power
Throttling, Step Loading (Poppet), Slide Valve w/Var. Vi, VFD w/Var. Vi and Ideal
110.00
100.00
90.00
80.00
70.00
60.00
50.00
40.00
30.00
20.00
10.00
0.00
Throttling
Poppet
Slide Valve
VFD
Ideal Eff.
0%
20%
40%
60%
% Capacity
80%
100%
Improved Part Load Efficiency
Solution
Capacity 20% -100%
(720 RPM 3600 RPM )
100% Slide Valve
Base Loaded, Capacity 100%
3600 RPM
100% Slide Valve
The improvement in part load efficiency will vary based on…
Compression Ratio
Minimum Speed
Volume Control
Compression Ratio (CR)
194.8psiaD / 48.7psiaS
4:1 CR
194.8psiaD / 19.48psiaS
10:1 CR
Example: Improvement at 40% capacity by speed
over 40% capacity by slide valve
4:1 CR = 19% Improvement
10:1 CR = 33% Improvement
Minimum Speed
• Minimum speed established by the limitations of the:
•compressor
•motor
•oil type
•refrigerant.
NH³
Compressor Minimum Speed
•Sleeve bearings ≥ 1800 rpm.
• Rolling element bearings ≥ 720 rpm
• Viscosity required at 1800rpm is ≥
11cSt.
• Viscosity required at 720rpm is ≥
18cSt.
Speeds below design limits may
cause a mixed film or boundary
lubrication condition.
Oil & Refrigerant
R-717@ 20˚F (-6.5˚C)SST
720 RPM Minimum
R-22 @ 10˚F (-12˚C)SST
1250 RPM Minimum
R-22 @ 20˚F (-6.5˚C)SST
1500 RPM Minimum
Motor Minimum Speed
• An issue of bearings and heat
rejection.
• Air-flows at speeds below
50% speed may not be
adequate.
• Check with manufacturer for
minimum speed.
• Motor stator temperature
protection is now required
(NEC 2005).
Fractional HP Squirrel Cage
Fan/s Ensure Adequate Air
Flow At All Speeds
Variable Volume Control
5.0
5.0
With VSD at part load
Vi stays ideal
4.0
Vi 3.0
Vi at part load
by slide valve
2.2
2.0
1.0
0
15 20
40
60
% Capacity
80
100
% Capacity to
% Slide Valve Position
Energy Savings @ Part Load
Slide Valve w/Variable Vi
VFD w/Variable Vi
HP to $$$ - 10HP may cost over $5,200 annually (.08 KWH).
RWFII-270 @ 20˚F (-6.7˚C) SST / 95˚F (35˚C) SDT 4.06:1 CR on Ammonia
Horse Power
Slide Valve, VFD and Ideal Part Load Efficiencies
700.00
650.00
600.00
550.00
500.00
450.00
400.00
350.00
300.00
250.00
200.00
150.00
100.00
50.00
0.00
0%
S.V. Control
650HP
VFD Control
19
HP
52
HP
107
HP
20%
40%
60%
% Capacity
80%
100%
RWFII-270 @ 20˚F (-6.7˚C) SST / 75˚F (24˚C) SDT / 2.9:1 CR on Ammonia
Horse Power
Slide Valve, VFD and Ideal Part Load Efficiencies
500.00
450.00
400.00
350.00
300.00
250.00
200.00
150.00
100.00
50.00
0.00
0%
475HP
S.V. Control
VFD Control
9
HP
31
HP
62
HP
20%
40%
60%
% Capacity
80%
100%
RWFII-270 @ -40˚F (-40˚C) SST / 95˚F (35˚C) SDT / 18.75:1 CR on Ammonia
Horse Power
Slide Valve, VFD and Ideal Part Load Efficiencies
450.00
400.00
350.00
300.00
250.00
200.00
150.00
100.00
50.00
0.00
420HP
S.V. Control
VFD Control
26
HP
82
HP
137
HP
0%
20%
40%
60%
% Capacity
80%
100%
RWFII-270 @ -40˚F (-40˚C) SST / 75˚F (24˚C) SDT / 13.45:1 CR on Ammonia
Slide Valve, VFD and Ideal Part Load Efficiencies
350.00
Horse Power
300.00
S.V. Control
335HP
VFD Control
250.00
18
HP
200.00
150.00
60
HP
100.00
50.00
102
HP
0.00
0%
20%
40%
60%
% Capacity
80%
100%
Typical VSD Operation
Slide Valve Unloaded – NH3
Percent Slide Valve
Percent
Valve
Capacity
Control Strategy
Strategy
Capacity Control
Speed
100%
100%
90%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
0%
SV
Speed
Off
0%
0%
Run
20%
20%
40%
60%
40%
60%
Percent
Percent Speed
Speed
80%
80%
100%
100%
Typical VSD Operation
Slide Valve Loaded – NH3
Capacity Control Strategy
Percent Slide Valve
100%
Speed
SpeedRun
SV
Off
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
0%
20%
40%
60%
Percent Speed
80%
100%
Typical VSD Operation
with Standby Mode (Vyper™)
Capacity Control Strategy
Percent Slide Valve
100%
Stand-by
Speed Run
at 100%SV
90%
80%
Speed
Speed
Up To
2Up
HrsTo
2 Hrs.
70%
60%
50%
40%
30%
20%
10%
0%
0%
20%
40%
60%
Percent Speed
80%
100%
Economizing
Econo. Improvement of HP/TR -40F/95F
14
12
HP/TR
10
Non E SV
8
Econ VSD
Non E VSD
6
Econ SV
4
2
0
0
50
100
Tons
150
Controlling The VSD
Manufacturers Controller To Be The Supervisory
Control
Low Oil Flow
High Motor Amps
High discharge Pressure
Low Mass Flow
The PLC Controls The Speed But Is Routed Through
The Package Mounted Controller
VSD Control with Quantum LX
Capacity Control Setup
Motor & Drive Setup
Skip Frequencies
Speed trend line
Can I Retrofit
My Existing Equipment???
VSD?
Retrofitting Existing Equipment
For Energy Savings
• Ensure the VSD is constant
torque.
• Motor is inverter duty rated.
• Minimum speed is identified.
• Ensure environment is
adequate.
• Total cable distance between
drive and motor is within spec.
• Controller is able to protect
the compressor as well as
control the drive.
To VSD or Not To VSD?
Qualifying Expense Through Savings (ROI)
Total Load Profile
1.
2.
3.
4.
Hours of Operation?
Compression Ratio?
Cost per KWH?
Part Load Profile?
How Do I Accurately Get This Information?
Items 1-3 Are Easy
What is My
Part Load
Profile?
Quantum™ LX Load Profile Screen
The VSD Energy Savings Calculator
The Load Profile
Data is Entered
Payback in 1.8 yrs.
What About Step Loading Machines?
Vyper™ Retrofit At JCI-Frick
in Waynesboro, PA
In Conclusion………..
When Properly Applied, VSDs On Rotary Screw
Compressors Can:
Lower Energy Usage & Cost
Enhance Economizer Boost
Remove Setpoint Over-Shoot
Provide Quicker Response To Load Fluctuation
Keep Capacity Slides Fully Loaded
Reduce Maintenance Costs
Reduce Noise Levels
Properly Applied Is The Key!
Thank You!